CA1289316C - Method of curing a compressible printing blanket and a compressible printing blanket produced thereby - Google Patents

Abstract

Abstract of the Disclosure A laminated printing blanket having compressible and resilient properties and a method for its production are provided. The compressible characteristics are provided by disposing an intermediate layer having substantially uniformly distributed voids of substantially uniform size between the substrate layer and the surface layer of the printing blanket. The voids of the compressible intermediate layer are formed by introducing microcapsules into an elastomeric compound formulated for production of the intermediate layer, and by vulcanizing the intermediate layer at a temperature below the melting point of the microcapsules to a degree sufficient to fix the microcapsules in place within the structure of the intermediate layer. A final vulcanizing step completes the curing of all layers to produce a laminated unitary blanket.

Description

METHOD OF C~RING A COMPRESSIBLE PRINTING BLANKET
~ND A Co~PRESSIBLE PRINTING BLANKET PRODUCED THEREBY
This invention relates to a method of curing printing blankets and printing blanket produced thereby, and in particular ralates to a compreseible printing blanket of the type uged in offset lithographic printing.
The u8~ of blankets in offset lithograpy i8 well known and ha~ a primary function of tran~ferring ink from a printing plate to paper. Printing blanket~ are ~ery carefully designed 80 that the surface of the blanket is not damaged either by the mechanical contact of the blan~et with the part~ of the press or by chemical reaction with the ink ingredients. Repeated mechanical contacts cause a certain amount of compression of the blanket which must be within acceptable limits 80 that the image is properly reproduced. It is also important that the blanket have resiliency, i.e. be capable of eventually returning to its original thicknes6, and that it provide constant image transfer regardless of the amount of use to which it is put.
Printing blankets are normally composed of a substrate base material which will give the blanket integrity. Woven fabrics are preferred for this base.
The bas0 may consist of one, two, three, or more layers of fabric. The working surface, by which is meant the surface that actually contacts the ink, is usually a layer of elastomeric material ~uch as rubber. The blanket is conventionally made by calendaring or spreading rubber in layers until a desired thickness of rubber has been deposited, after which the assembly is cured Ol vulcanized to provide the finlshed blanket. Such a blanket is acceptable for many applications, but often lacks the nece66ary compressibility and re~iliency needed for other ' ~9~6 applications. It i8 de~irable, ~herefore, to produ~e more highly compreffsible blankets with improved resiliency.
It is difficult to obtain an ~mproved compressibility factor by the standard construction described above because the rubber material, while it i8 highly elastomeric, i8 not compres~ible and cannot be compressed in a direc~ion at right angle~ to its surface without causing a di~tortion or stretch of the blanket in areas adjacent to the point of compression. If irregularitie~ exist in ehe printing plate, the presseR, or the paper. the compression tO which the blanket is expo~ed will vary during the printing operation and the irregularities in the plate~, presses, or paper will be magnified by the lack of compression in the printing blanket.
The key to providing a printing blanket having the desired compressibility and resiliency i6 in providi~g a compressible layer therein. In particular, it has been found that by including at least one layer of material comprising a compressible layer of resilient polymer in a printing blanket that erinting problems such as those described above as well as "blurring~ (lack of definition), caused by a small standing wave in the blanket printing surface adjacent to the nip of the printing press, can be avoided. Also, a compre~sible layer can 6erve to absorb a "6mash", that is a substantial deformation in the blanket caused by a temporary increase in the thickne~s in the material to be printed, for example, the accidental introduction of more than one sheet of paper during the printing operation. By incorporating a compressible layer in the blanket, a "6mash" can be ab60rbed without permanent damage to the blanket or impairment to the printing quality of the blanket. In addition, a re~ilient, compressible layer hel2s to maintain the evenne~s of ~he printing ~urface and the thickne~s of the blanket during the printing operation by restoring the nor~a~ thickne~s of the blanket ater compression at the nip of the pre~.
Many different means of producing a compressible layer within a pr~nting blanket are known in the art. ~or example, compre~ible layers have been formed by mixing granular salt particle~ with the polymer u~ed to prcduce the layer, and thereafter leaching the salt f~om the polymer to create void~. The voids in the layer make possible the positive displacement of the surface layer without distortion of the surface layer since volume compression occur~ and displacement takes place substantially perpendicular to the impact of the press.
Such a method is disclosed in Haren et al, U.S. Patent 4,025,685. Other method~, such as using compressible fiber structures have been used heretofore to produce compressible layer~. Examples are found in Duckett et al, U.S. Patents 3,887,750 and 4,093,764. Rodriguez, U.S.
Patent 4,303,721, teaches a compressible blanket made using blowing agents to create voids in the compressible layer. The use of rubber particles to create voids is disclosed in Rhodarmer, U.S. Patent 3,795,568.
The forming of voids using blowing agents has the disadvantages that the size of the voids formed, and the interconnecting of the voids is not easily controlled.
Oversizsd voids and interconnected voids cause some areas of the printing blanket to be more compressible and less resilient than adjacent areas of the printing blanket, which re6ults in deformations during printing. The leaching of salt~ from a polymer matrix has the disadvantages that the particle sizes u~ed are limited, o~

and that the leaching 8tep i8 difficult, time con~uming and expensive.
More recently, it ha~ been found pre~erable to produce printing blanket~ having a compres~ible layer comprising a cellular re~ilient polymer having cells or voids in the compressible layer in ~he form of di6crete microcell~. It has been found particularly advantageou~
to produce a compre~sible layer by incorporating hollow microcapsules in the polymer, as illustrated by Shrimpton et al, U.S. Patent No. 3,700,541, and corresponding Briti6h patent 1,327,758; and by Larson, U.S. Patent 4,042,743.
In prior art methods of producing a compressible lay~r for a printing blanket employing microcapsules, it has been found that the thickness of the compre6sible layer formed iB not easily controlled since microcapsules most suitable for this use will melt at a temperature lower than the vulcanizing temperature used for vulcanizing the printing blankets. Since the microcapsule6 melt before the vulcanization i6 complete, and before the compre6sible layer achieve6 a ~et 6tructure, agglomeration of the voids created by the microcapsules occurs, and size variations in the voids also occur. This can affect overall performance ~5 properties of the blanket. Al~o, the variation~ in the sizes of the voids can weaken the printing blanket and cause the printing blanket to wear out prematurely.
It is a feature of the present invention to overcome the deficiencies in the prior art printing blanket6 described above. The present invention provide6 a me~hod of making a laminated printing blanket by forming a base ply and a surface layer and dispo~ing therebetween an intermediate compres6ible layer formed by incorporating microcap~ule6 in an elastomeric matrix.

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In accoedance with one aspect of the method of the present invention, at least one fabric substrate layer iB provided. An inte~mediate compres6ible layer of an elastomeric material i8 formed thereon. The elastomeric material has a substantially uniform thickness and has microcapsules incorporated therein and su~tantially uniform distributed throughout the compressible layer.
The compressible layer i8 maintained at a temperature below the melting point of the microcapsules for a time ~ufficient to cause the elastomeric material to vulcanize to a degree sufficient to substantially fix the positions of the microcapsules within the compressible layer. A
surface layer iB provided over the compressible layer to form a printing blanket construction which is vulcanized to cure all of the layers and provide the compressible layer with sub6tantially uniformly distributed void6 of substantially uniform size. In a preferred form, the elastomeric material including the microcapsules iB
initially fixed by adding an accelerator capable of permitting vulcanization of the elastomeric material at a temperature ranging from 43 to 77C (110F-170F), over a time period ranging from 1 to 12 hours. These temperature conditions, which are below the melting point of the microcapsules, cau~e an initial vulcanization of the elastomer compound to form the intermediate layer. Then all of the layers are laminated to create a unitary printing blanket by finally vulcanizing the assembly under controlled heat and pressure. The final product comprises a laminated printing blanket including at least one substrate layer, a surface layer, and a compres~ible intermediate layer disposed therebetween. The intermediate layer ha~ a closed-cell cellular structure having substantially uni~orm thickness and sub~tantially evenly distributed void6 of 6ubstantially uniform 6ize, in which the void~ a~e not interconnected. Additional reinforcing fabcic layer~ may also be incorporated therein.
Othe~ details, features, ob~ect~, uses and advantages of this invention will become apparent from the embodiments thereof presented by the way of example in the following detailed description, the accompanying drawings, and the appended claims.
In o~der that the invention may be more readily under~tood, reference will now be made by example to the accompanying dcawings in which:
Fig. 1 illustrate~ a cross section of a printing blanket of the present invention, indicating the relationship of the layers; and Fig. 2 iB a flow diagram, illustrating the stees comprising the method of the present invention.
Reference is now made to Fig. 1 which illustrates a printing blanket made according to the present invention. The printing blanket 10 comprises a surface layer 20, a reinforcing fabris layer 22, a compressible layer 24, and at least one additional layer of fabric as a substrate or base. For purpose~ of illustration, two substrate fabric layers 26 and 27 aLe shown in the drawing. Those skilled in the art will recognize that the number and types of layers used can vary depending on the uses intended. Adhesive layers 30, 32, 34 and 36 are preferred to ensure sufficient bonding between the different layers in the blanket. Voids 28 in the compressible layer 24 make possible di~placement of the surface layer 20 without distortion thereof under operating conditions. As illustrated in the drawing, the voids 28 in the compressible layer 24 have sub6tantially uniform size and subtantially uniform distribution and are not inter-connected. It ha6 been found that the 1;~89~ 6 dimen~ions of the voids 28 produced in the comeressible layec 24 are qenerally in ~he same range as the dimen6ions of the microcapsule~ u~ed to create the voids. In general, the voids will be in the range of 10-125 microns in diameter, and preerably 20-60 microns.
The compr2ssible layer 24 is formed by an elastomeric material having the usual processing, stabilizing, strenqthening and curing additives and is formulated with reference to its specific application. A~
is known in the art, this formulation i~ different from the one used for the surface layer 20. Any suitable polymeric material which i~ considered a curable or vulcanizable material can be used for example, natural rubbee, styrene-butadiene rubber (SBR), EPOM
(ethylene/propylene/non-conjugated diene ter-polymer rubber), butyl rubber, butadiene, acrylonitrile rubber (NBR), polyurethanes etc. An elastomer which is resi6tant to solvents and ink is preferable.
The adhesive layers 30, 32, 34 and 36 may be any suitable elastomeric adhesive known in the art.
Preferably the adhesive will be a rubber cement. The fabric layers 22, 26 and 27 should be made of plain woven fabric of lower extensibility in the warp direction (in the direction longitudinal to the machining of the blanket), and are typically high grade cotton yarns, which are free from slubs and knots, weaving defects, seeds, etc. The fabeic may also be rayon, nylon, polye6ter, or mixtures thereof. When aæplying adhesive to any of the fabric layers, it is usually spread with a knife-over-roll-spreader. The adhesive i8 apelied in layers until the desired thickness is obtained.
Typically, a fabric layer will be about 0.076 to 0.41 mm (0.003 to 0.016 in.) thick.

The compres~ible layer 24 is formed by applying the elastomeric compound as described above, containing microcap~ules, to the fabric substrate 26, also using a knife-over-roll-spreader. The elastomeric compound is brought to the desiIed consistency ~or spreading by adding a ~olvent. In genesal, a number of layer~ of the compound are required to make an intermediate layer 24 of the de~ired thickne~. As each layer is applied, it is solidified, but not crosslinked, due to the evaporation of the solvent. In general, the compressible layer will be about 0.20 to 0.38 mm (0.008 to 0.015 in.) thick. It is preferred that the layer be about 0.2B to 0.30 mm (0.011 to 0.012 in.) thick.
The surface layer 20 is made usinq the 6a~e lS procedure as is described in the making of the intermedia~e layer, using an elastomeric compound suitable for the working face of the printing blanket 10. In general, a number of layers of compound are required to make a surface layer 20 of the desired thickness. In general, the surface layer 20 will be about 0.005 to 0.25 in. thick, preferably be about 0.010 to 0.015 in. thick.
It is most preferred to provide a surface layer in the range of about 0.30 to 0.38 mm (0.012 to 0.015 in.) thick.
According to the present invention, conventional re~inous microcapsules that are known in the art may be used in the intermediate layer. Any microcapsules having the properties described herein will be suitable for use in the present invention. Microcapsules having a melting point of about 74 to 132C (165F to 270F) can be used.
Preferably, the microcapsules will melt at about 8Z
(180F). Some of ~he materials suitable for use in the microcapsules are phenolic re6in, and thermoplastic materials such as polyvinylidene chloride. Preferably, ? ~1 ~

the material~ used in making the microcapsules will be thermoplastics. Example6 of such materials are polyvinylidene chloride, poly methyl methacrylate, polyvinyl chloride, polyacrylonitril2, and copolymers thereGf. Preferably, a copolymer of acryloni~rile and vinylidene chloride will be used. Mixtures of different kinds of thermoplastic microcapsules may be used if desired.
In addition, any of the resins having the properties that are described herein that are mentioned in U.S. Patent 2,797,201 may be used. Of course, it is understood that these materials named are illustrative, and the present invention i6 not limited thereby.
The microcapsules used in the method claimed are approximately seherical in shape and range from 10 to 100 micron6 in diameter, with an average particle size of about 30-50 microns. These capsules are formed, for example, with a thin, elastic, thermoplastic shell which may be composed of vinylidene chloride/acrylonitrile copolymer or similar material. These cap~ules may be formed as i8 generally de~cribed in U.S. Patent 2,797,201 or U.S. Patent 3,615,972. The capsules may contain an inert gas to maintain their shape. The capsules may be mixed with the elastomeric matrix in any conventional method, such as dispersing them in a dough of the elastomer. The capsules are dispersed by mixing them uniformly throughout the elastomeric matrix for a controlled time period. Because the act of mixing generate~ a certain amount of heat, it is preferred not to mix longer than 30 minute~. Studies made utilizing an electron micro6cope, of the preferred embodiment, appear to indicate that a major portion, if not substantially all of the microcap~ules u6ed in making the compressible layer are recognizable in the completed printinq blanket when utilizing the method of the eresent invention.
The a~ount and the size of the specific capsules u6ed may be based on the desired compres~ibility of the blanket. For example, microcapsule~ having an average diameter of 40 micIons may be used in a catio representing 50 percent of the ela~to~eric material used in the compressible layer to produce a compressible layer having 50 percent voids.
Preferably the presence of water is avoided during the incorporation of the microcap~ules in the elastomer in order to avoid water vapor blowing during any subsequent heating of the polymer. For this reason, the microcapsules are preferably dried before mixing the elastomer.
Reference is now made to Fig. 2 which is a flow diagram illustrating the steps in one method of preparing the printing blanket of the invention. The reference numbers used in Fig. 2 are the same as those used in Fig.

2~ 1 where the same elements are described. As noted in the flow diagram adhesive layer 34 is spread on the upper surface of substrate layer 26; compressible layer 24 i~
formed by mixing microcapsules and an activator or accelerator with an elastomeric compound for about 30 minutes: and the compound is spread onto the fabric substrate 26. In general, a number of layers of compound are required to obtain the desired thickness of the compressible layer. A layer of adhesive 32 is then spread on the upper surface of the layer 24, and the fabric layer 22 as bonded to the layer 24. The resulting assembly is then subjected to sufficient heat to initially vulcanize layer 24 to a degree sufficient to set the structure of the polymer ma~rix with the microcap6ules fixed in position therein, 6aid vulcanization being made po6sible by the addition of an accelerator that promote~ the vulcanization at a low temperature. Vulcanization at 56C
(150F) for ~ hours is preferred, however, a temperature range of 43 to 77C (110 to 170F) for period~ of 1 to 12 hours may also be used. Theoretically, ambient temperatures for longer periods of time are also pos6ible. The preferred accelerator ~eferred to in this example iB a dithiocarbamate such a6 the one 601d under the name "Butyl-Eight" (trademark), which is available from R.T. Vanderbilt Co. Other examples of accelerators u6eful in this invention are piperidinium pentamet ffllene dithiocarbamate (Accel 552 (trademark), available from Dupont Corp), zinc dibenzyl dithiocarbamate (Arazate (trademark), available from ~niroyal), zinc dibutyl dithiocarbamate (Butyl Ziram (trademark), available from Pennwalt). Thi~ low temperature vulcanizing 6tep create6 a permanent cell structure which iB not changed throughout the rest of the manufacturing process.
In the preferred embodiment of the invention, the low temperature initial vulcanization, using the acccelerator, will cause optimum final vulcanization of the csmpressible layer. That is, substantially all the ~ites in the polymer, which are most 6usceptible to cros61inking, are cro~61inked in thi6 6tep, said crosslinking providing the preferred ela6tic modulus and re~iliency and other elastic properties of the elastomer.
Of cour6e, tho6e 6killed in the art will recognize that, in a rubber product, cros61inking i6 a continuing proces6, and that no rubber material i6 ever completely cro6slinked. Therefore, those skilled in the art will recognize that the vulcanization in the low temperature cure of the compressible layer may be interrupted prior to optimum vulcanization as long as the elastomeric matrix containing the microcapsules has ~et up ~ufficiently to "freeze~ the microcap~ules in po~ition and still obtain an acceptable product. A thus ~partially~ vulcanized compressible layer may obtain better cros~linking with the base layer and the printing surface. One skilled in the art will al~o recognize, however, that a compres~ible layer, which has been substantially completely vulcanized, may be cro~linked to the base layer and the surface layer by means of an adhesive specifically formulated for that purpose.
After curing comeressible layer 24, a second sub~trate fabric layer 27 is laminated to the lower surface of layer 26, by means of adhesive layer 36, on laminating pinch collers. Following this step, the working surface layer 20, compounded as described above, is applied to the upper surface of reinforcing fabric layer 22, using adhe~ive layer 30 to achieve a bond. The resultant assembly i~ then subjected to the final vulcanization process known in the art, at a temperature range of 132 to 160C (270F to 320F), and preferably 143 to 149~ (290F to 300F), for one-half hour to 16 hours under pressures ranging from atmospheric to 5.2 bars (75 psi). These variables will depend on the exact compounding.
In the final vulcanizing ~tep, paper having a fine finish i8 disposed in contact with the face of the printing blanket, together with a fine talc prior to placing the blanket in the vulcanizing oven. The paper, contacting the printing blanket surface, assures the smoothne6s of the printing blanket, since the smoothness of the paper is imparted to the printing blanket. For most applications, the finish 80 provided to the printing blanket will be sufficient for its use, and grinding of the surface will not be required.
It has been found that the ~tep of ~ubjecting the intermediate layer to a partial vulcanization or semi-cure causes the microcapsules to be captured in stationary or ~et po~itions in the elastomeric matrix. Since the po6itions of the microcap~ule~ are ~et in the matrix, the portions of the void~ created br the microcapsules are predetermined by the position of the microcap~ules in the matrix. When the assembled blanket undergoes the final vulcanization ~tep, the already set structure of the in~ermediaee layer holds its shape and prevents the agglomeration of void6 or the collapse of voids in the layer. Thi6 fixed position will not change under final processing of the blanket.
It has been pointed out that the melting temperature of the microcapsule6 is below the final vulcanizing temperature. Obviously most, if not all of the microcap~ule6 probably melt, but since they are retained within closed cells, they appear to re-form, and act as a coating of the walls of the voids. It is al~o possible that some of the material will partially interact with the elastomer to form a new coating substance.
Electron micro~cope studies have confirmed that the thermoplastic material from which the microcapsules are made remain the voids of the completed printing blanket.
Of course, as long as the voids in the compres6ible layer are formed as de~cribed, the presence or absence of the microcap~ule material in the voids will not affect the performance of the blanket. Regardles6 of what happen~
the previously fixed void structure remain6 unchanged.
The exact construction of the blanket may be varied according to its final use. For example, a ~ingle fabric ~ubstrate layer 26 may be utilized without layer 27, or a third or additional similar layers may be incorporated. It may also be desired to provide additional reinforcing fabric layers similar to layer 22.
Since, Por production purposes, it i8 alBO
po6sible to prepare a compre6sible layer for u~e as an intermediate layer in a printing blanket in one location, and to ~hip the compressible layer to another location for fabrication of the blanket, the comeressible layer, by itself, represents a separate feature of the of the instant invention. The compressible intermediate layer may be prepared by the same method decribed above in the description of the preparation of the printing blanket, and will have generally the same paramaters with regard to dimen6ions and distribution of voids. In such a case, it would be preferable to aeply the compressible layer to at least one fabric substrate layer prior to vulcanizing and shipping. However, it is also possible to utilize a fabric or release paper as the sub6trate layer and reinforcing layer and strip off the compressible layer for shipping. In that case, the6e layers will be added later.
Also, it will be recognized by those skilled in the art that it is po6sible to make a complete blanket construction, including a compressible layer containing microcapsules and a dithiocarbamate accelerator, and subject the construction to a low temperature vulcanizing step in order to fix the microcapsules in the compressible layer as before, before 6ubjecting the blanket construction to final vulcanization.
It i8 to be noted that the above description i~
merely illustrative of the invention, and other parameter6 and embodiments may be used without departing from the inventive concept herein. Accordingly, the pre6ent invention is only limited by the appended claims.

Claims (19)

1. A method of making a laminated printing blanket construction comprising the steps of:
providing at least one fabric substrate layer, forming an intermediate compressible layer of an elastomeric material thereon, said intermediate compressible layer having a substantially uniform thickness and having microcapsules incorporated therein, said microcapsules being substantially uniformly distributed throughout said intermediate compressible layer, maintaining said intermediate compressible layer at a temperature below the melting point of said microcapsules for a time sufficient to cause said elastomeric material to vulcanize to a degree sufficient to substantially fix the positions of said microcapsules within said intermediate compressible layer, providing a surface layer over said intermediate compressible layer to form a printing blanket construction, and vulcanizing said construction to cure said layers and provide said intermediate layer with substantially uniformly distributed voids of substantially uniform size.

2. A method as claimed in claim 1 including the step of adding an accelerator to said intermediate compressible layer during formation thereof, said accelerator being capable of promoting vulcanization of said intermediate compressible layer at temperatures of between about 43 to 77 degrees C in from about 1 to 12 hours.

3. A method as claimed in claim 1 wherein the formation step for said intermediate compressible layer includes the steps of forming said elastomeric material, incorporating said microcapsules therein to form a mixture, spreading said mixture onto said fabric substrate layer, and applying a fabric reinforcing layer over the upper surface of said intermediate compressible layer.

4. A method as claimed in claim 1 in which said construction is vulcanized at a temperature of about 132°
to 160 degrees C.

5. A method as claimed in claim 2 in which said accelerator is a dithiocarbamate.

6. A method as claimed in claim 1 in which said microcapsules are formed of a thermoplastic resin.

7. A method as claimed in claim 2 in which said accelerator is a dithiocarbamate and said microcapsules are of a copolymer of acrylonitrile and vinylidene chloride.

8. A laminated printing blanket construction comprising at least one substrate layer, a surface layer, and an intermediate compressible layer having a substantially uniform thickness, said intermediate compressible layer comprising an elastomeric material and having a cellular structure with a plurality of closed cells forming voids, said voids being of substantially uniform size and being substantially uniformly distributed throughout said intermediate compressible layer, said voids being formed by distributing microcapsules and an accelerator capable of causing vulcanization of said elastomeric material at a temperature below the melting point of said microcapsules within said intermediate compressible layer, and maintaining said intermediate compressible layer at a temperature below the melting temperature of said microcapsules for a time sufficient to cause said elastomeric material to vulcanize to a degree sufficient to substantially fix the positions of said microcapsules within said intermediate compressible layer.

9. A printing blanket as claimed in claim 8 in which said voids have a diameter in the range of from about 10 to 125 microns.

10. A printing blanket as claimed in claim 8 in which said microcapsules are formed of a thermoplastic resin.

11. A compressible layer for use in a laminated printing blanket construction comprising an elastomeric material having a substantially uniform thickness and having a cellular structure with a plurality of closed cells forming voids, said voids being of substantially uniform size and being substantially evenly distributed throughout said compressible layer, said voids being formed by distributing microcapsules and an accelerator capable of causing vulcanization of said elastomeric material at a temperature below the melting point of said microcapsules within said compressible layer, and maintaining said compressible layer at a temperature below the melting temperature of said microcapsules for a time sufficient to cause said elastomeric material to vulcanize to a degree sufficient to substantially fix the position of said microcapsules within said compressible layer.

12. A compressible layer as claimed in of claim 11 in which said voids have a diameter in the range of from about 10 to 125 microns.

13. A compressible layer as claimed in claim 11 in which said microcapsules are formed of a thermoplastic resin.

14. A method of making a compressible layer for use in a laminated printing blanket construction comprising the steps of:
forming said compressible layer of an elastomeric material and mixing microcapsules with said elastomeric material so that said microcapsules are substantially uniformly distributed therein, maintaining said compressible layer at a temperature below the melting point of said microcapsules for a time sufficient to cause said elastomeric material to vulcanize to a degree sufficient to substantially fix the positions of said microcapsules within said compressible layer, thereby providing a compressible layer having a substantially uniform thickness with said microcapsules being substantially uniformly distributed throughout said compressible layer.

15. A method as claimed in claim 14 including the step of adding an accelerator to said compressible layer during formation thereof, said accelerator being capable of promoting vulcanization of said compressible layer at temperatures of between about 43 to 77 degrees C in from about 1 to 12 hours.

16. A method as claimed in claim 15 in which said accelerator is a dithiocarbamate.

17. A method as claimed in claim 14 in which said microcapsules are formed of a thermoplastic resin.

18. A method as claimed in claim 14 in which said step of maintaining said compressible layer below the melting point of said microcapsules is carried out at a temperature of about 43 to 77 degrees C.

19. A method as claimed in claim 15 in which said accelerator is a dithiocarbamate and said microcapsules are of a copolymer of acrylonitrile and vinylidene chloride.